[c++-pthreads] Restating the Jason model

[Alexander Terekhov]

Jason Merrill wrote:
> 
> I think this is an appropriate time to restate my proposal.  I think
> there's a fair amount of consensus around these three points:
> 
>  * Cancellation is a normal exception.

Agreed.

>  * If a cancellation exception is destroyed, the cancellation request
>    is re-entered, and acted on again at the next cancellation point.

Disagreed.

>  * Cancellation is disabled during unwinding.

Only when propagating thread termination request exception (base class
for both thread_cancel_request and thread_exit_value<>). See also POSIX 
TC2 (Change Number: XSH/TC2/D6/7 [XSH ERN 77]).

> 
> But there are still some open questions:
> 
>  * Which of the POSIX cancellation points are cancellation points in C++?

All.

[...]
>  * Which bits of the C++ library are cancellation points?
> 
> I would think pretty much all I/O code, and nothing else.

I would think C++ library should have a lot of async-cancel-safe stuff.

[...]
>  * Should cancellation also be disabled in destructors run during normal
>    execution?  

Not by the implementation (2-phase EH aside for a moment).

>               In catch blocks?

That depends.

> 
> IMO, no and no.

See POSIX TC2 (Change Number: XSH/TC2/D6/7 [XSH ERN 77]).

> 
>  * How can C++ code interact with a cancellation exception?
> 
> I think everyone agrees that it should be possible to catch a cancel by
> name.  We still need to specify that name and any additional operations the
> cancel object might support.

Yes.

> 
>  * What about pthread_exit?

http://www.codesourcery.com/archives/c++-pthreads/msg00005.html

> 
> I'm happy with the g++ status quo whereby destroying a pthread_exit
> exception calls terminate.  

I'm not happy with that. Why should the g++ care what I do with *MY*
exceptions?

regards,
alexander.